In Enriched mode, Emacs distinguishes between two different kinds of newlines, hard newlines and soft newlines. You can also enable or disable this feature in other buffers, by typing M-x use-hard-newlines.
Hard newlines are used to separate paragraphs, or anywhere there
needs to be a line break regardless of how the text is filled; soft
newlines are used for filling. The RET (
open-line) commands insert hard newlines. The fill
commands, including Auto Fill (see Auto Fill Mode), insert only soft
newlines and delete only soft newlines, leaving hard newlines alone.
Thus, when editing with Enriched mode, you should not use RET
or C-o to break lines in the middle of filled paragraphs. Use
Auto Fill mode or explicit fill commands (see Explicit Fill Commands)
instead. Use RET or C-o where line breaks should always
remain, such as in tables and lists. For such lines, you may also
want to set the justification style to
(see Justification in Enriched Text).